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Digital Transmission & Analog Transmission. 1. DIGITAL-TO-DIGITAL CONVERSION. Digital Data -> Digital Signal Three techniques: line coding ( always needed ) block coding (working with NRZ-I) Scrambling (working with AMI). Figure 4.1 Line coding and decoding.

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slide2

1. DIGITAL-TO-DIGITAL CONVERSION

  • Digital Data -> Digital Signal
  • Three techniques:
    • line coding (always needed)
    • block coding (working with NRZ-I)
    • Scrambling (working with AMI)

4.#

slide4

Figure 4.2 Signal element versus data element

r = number of data elements / number of signal elements

slide5

Data Rate Vs. Signal Rate

    • Data rate: the number of data elements (bits) sent in 1s (bps). It’s also called the bit rate
    • Signal rate: the number of signal elements sent in 1s (baud). It’s also called the pulse rate, the modulation rate, or the baud rate.
  • We wish to:
  • 1. increase the data rate (increase the speed of transmission)
  • 2. decrease the signal rate (decrease the bandwidth requirement)
    • Worst case, best case, and average case of r
    • S = c * N / r baud
slide6

Baseline wandering

Baseline: running average of the received signal power

DC Components

Constant digital signal creates low frequencies

Self-synchronization

Receiver Setting the clock matching the sender’s

slide12

High=0, Low=1

  • No change at begin=0, Change at begin=1
  • H-to-L=0, L-to-H=1
  • Change at begin=0, No change at begin=1
slide14

Multilevel Schemes

  • In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln
  • m: the length of the binary pattern
  • B: binary data
  • n: the length of the signal pattern
  • L: number of levels in the signaling
slide18

Block Coding

  • Redundancy is needed to ensure synchronization and to provide error detecting
  • Block coding is normally referred to as mB/nB coding
  • it replaces each m-bit group with an n-bit group
  • m < n
slide22

Scrambling

  • It modifies the bipolar AMI encoding (no DC component, but having the problem of synchronization)
  • It does not increase the number of bits
  • It provides synchronization
  • It uses some specific form of bits to replace a sequence of 0s
slide23

Figure 4.19 Two cases of B8ZS scrambling technique

B8ZS substitutes eight consecutive zeros with 000VB0VB

slide24

Figure 4.20 Different situations in HDB3 scrambling technique

HDB3 substitutes four consecutive zeros with 000V or B00V depending

on the number of nonzero pulses after the last substitution.

slide25

2. ANALOG-TO-DIGITAL CONVERSION

  • The tendency today is to change an analog signal to
  • digital data.
    • pulse code modulation
    • delta modulation.
slide27

According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency contained in the signal.

What can we get from this:

1. we can sample a signal only if the signal is

band-limited

2. the sampling rate must be at least 2 times the highest frequency, not the bandwidth

slide29

Contribution of the quantization error to SNRdb

SNRdb= 6.02nb + 1.76 dB

nb: bits per sample

(related to the number of level L)

The minimum bandwidth of the digital signal is nb times greater than the bandwidth of the analog signal.

Bmin= nb x Banalog

slide30

DM (delta modulation) finds the change from the previous sample

Next bit is 1, if amplitude of the analog signal is larger

Next bit is 0, if amplitude of the analog signal is smaller

slide31

3. TRANSMISSION MODES

1. The transmission of binary data across a link can be accomplished in either parallel or serial mode.

2. In parallel mode, multiple bits are sent with each clock tick.

3. In serial mode, 1 bit is sent with each clock tick.

4. there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.

slide33

4. DIGITAL-TO-ANALOG CONVERSION

Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data.

slide36

Data element vs. signal element

    • What is a signal element here?
  • 2. Bit rate is the number of bits per second.
  • 2. Baud rate is the number of signal elements per second. 3. In the analog transmission of digital data, the baud rate is less than or equal to the bit rate.
  • S = N x 1/r baud r = log2L
slide37

Figure 5.3 Binary amplitude shift keying

B = (1+d) x S = (1+d) x N x 1/r

qam quadrature amplitude modulation
QAM – Quadrature Amplitude Modulation
  • Modulation technique used in the cable/video networking world
  • Instead of a single signal change representing only 1 bps – multiple bits can be represented buy a single signal change
  • Combination of phase shifting and amplitude shifting (8 phases, 2 amplitudes)
slide45

5. ANALOG AND DIGITAL

Analog-to-analog conversion is the representation of analog information by an analog signal.

Modulation is needed if the medium is bandpass in nature or if only a bandpass channel is available to us.

Example: radio stations

slide47

Figure 5.16 Amplitude modulation

The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B.

slide50

Figure 5.19 FM band allocation

The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B. β has a common value of 4

slide51

Figure 5.20 Phase modulation

The total bandwidth required for PM can be determined from the bandwidth and maximum amplitude of the modulating signal:BPM = 2(1 + β)B.

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